The assembly of ribonucleoprotein (RNP) complexes is central to processes such as protein translation, mRNA splicing, and telomere maintenance. ATPases of the DEAD-box family are ubiquitous, highly conserved enzymes that play essential roles during RNP assembly in all kingdoms of life. DEAD-box proteins play critical roles in human health and disease; defects in DEAD-box proteins underlie the progression of specific cancers as well as developmental disorders, and are co-opted by RNA viruses such as HIV and West Nile for viral RNA processing. Though they are part of the SF2 family of helicases, DEAD-box proteins lack key domains present in processive SF2 helicases, and rely instead on trans factors that regulate ATP hydrolysis and substrate binding. During RNP assembly, ATPase activity has been proposed to drive the remodeling of secondary and tertiary RNA structures, coordinating the ordered addition of proteins to form functional RNP assemblies. The requirement of trans modulators makes DEAD-box ATPases ideal regulators, integrating RNP biogenesis with cellular signaling. However, due to the transient nature of their interactions, we have no molecular understanding of the how DEAD-box proteins engage and remodel their RNP assembly substrates. This proposal describes a hybrid approach to define the molecular details of four essential DEAD-box proteins (Dbp10, Drs1, Spb4 and Mak5) during the assembly of a complex RNP, the large ribosomal (60S) subunit. We genetically manipulated yeast strains to trap and enrich distinct, transient DEAD-boxRNP intermediates. The structural characterization of these dynamic complexes by cryo-electron microscopy, as part of an integrated approach that includes cross- linking mass-spectrometry and targeted in vitro reconstitution experiments, will shed light on the molecular interactions of DEAD-box proteins with substrate RNA and modulating co- factors. Because DEAD-box modulation of 60S maturation is closely associated with the regulation of nucleolar pre-60S release, we will use a color-switching yeast strain to probe the effect the expression of Dbp10, Drs1, Mak5 and Spb4 trapping mutants have on the subcellular distribution of 60s intermediates. Together, these studies represent a unique approach to understand the function of DEAD-box proteins in the centrally important 60S biogenesis pathway. These innovative reagents and their use within an integrative experimental approach will uniquely inform how DEAD-box proteins engage transient, dynamic intermediates to modulate RNP assembly.